For artificial legs there are both single- and multi-axis artificial knee joints. This invention is applicable to different types of artificial multi-axis knee joints for artificial legs, such as 4-, 5- and 7-axis knee joints, for the purpose of illuminating, but not restricting the invention, it will hereafter be described how the invention may be used for providing a 4-axis knee joint which has a means for controlling the bending characteristics of the knee joint. In comparison to single-axis knee joints, multi-axis knee joints have the advantage that better ground clearance and thereby improved safety during the use of an artificial leg may be obtained, and furthermore they enable movement while using a less energy-intensive step. Another advantage is that multi-axis knee joints have a higher stability during ground contact of the artificial leg than is the case for single-axis knee joints.
In common with many artificial multi-axis knee joints which are used in artificial legs, in addition to the links included in the knee joint, which links connect the upper and lower leg portions, and which enable pivoting of the upper and lower portions relative to each other, in the knee joint, they also have a mechanism for so-called swing phase control. This means that in use of the knee joint and during movement of the leg in the forward direction together with the subsequent swing in the forward direction of the artificial crus leg, the crus leg can be retarded by means of the bending characteristics of the knee joint to obtain a gentle and controlled motion of the leg when the user moves.
For this purpose an externally arranged hydraulic cylinder is often used which is arranged between the upper and lower leg portions of the knee joint, which hydraulic cylinder has the task of effecting the bending characteristics of the knee joint in such a way that the swinging of the artificial crus leg relative to the thigh leg is damped during the swing phase. The hydraulic cylinder is at one end thereof pivotally connected to the lower leg portion and at the other end thereof pivotally connected to the upper leg portion so that when bending the knee joint, the pivot motion of the knee joint is transmitted to a rectilinear motion of the piston of the hydraulic cylinder. The resistance of the hydraulic cylinder will thus determine the requisite torque for bending the knee joint.
However, previously known knee joints of the current type have a number of drawbacks. These are above all that such a knee joint provided with a hydraulic cylinder takes up a lot more space and further, the weight of the hydraulic cylinder contributes to a lower centre of gravity of the knee joint which is unfavourable during the use of the knee joint. Another disadvantage is that special attachment means are required for arranging the hydraulic cylinder on the knee joint. Furthermore, depending on geometrical conditions, in these knee joints the function of the hydraulic cylinder arranged on the knee joint for retarding the pivot motion of the knee joint is not adapted to the different phases of the movement in an optimal way, or in other words; the hydraulic cylinder gives a relatively high pivot resistance during a part of the walking cycle when the pivot resistance could advantageously be relatively low, and the hydraulic cylinder gives a relatively low pivot resistance, or no resistance at all, during a part of the walking cycle when the pivot resistance could advantageously be relatively high.